Directions For Macs: Click on the size file that you need. When it opens, go to "File" > "Save As" and save it into a folder you'll remember, or create one special or desktop art. Go to your "System Preferences" (usually the little white icon with a light switch) and in "Personal," click "Desktop and Screen Saver." Find the folder that says "Choose Folder," then find the folder you created for desktop art, and the file you saved. Select the file and voila!

Directions For Windows: Pretty similar to directions for Macs. We have first-hand experience with Macs, but we can try to help. :)